1. Field of the Invention
This invention relates generally to a printer with an automatic density adjusting function and a density adjusting method of the printer. More particularly, this invention relates to a printer with an automatic density adjusting function that prints a color image on a color photographic paper that has a cyan (C) layer, a magenta (M) layer and an yellow (Y) layer by producing a color of each layer and a density adjusting method of the printer.
2. Description of Related Art
A printer that prints an image shot by a digital still camera or the like on photographic paper and operates on the thermo-autochrome (TA) method is on the market. In TA method, color photographic paper (TA paper) that has C, M and Y layers itself produces the colors when it is heated and the produced colors are fixed when a light of a predetermined wavelength is thrown onto the TA paper. TA method does not require ink or toner.
When an image of a color with the same density is printed on different TA papers in the printer operating in TA method, the densities (print densities) of the color of the images printed on the TA papers are different due to differences of characteristics of devices of the printer and the differences of the characteristics of the TA papers and the changes of them with the passing of time.
To prevent the differences of the densities, a method has been proposed, in which test patterns that are references for print density adjustment are printed and the print densities are measured and the difference between the print densities and reference densities are fed-back as correction values and the printer is adjusted to achieve the reference densities.
Also, a printer has been proposed, which uses fixation fluorescent lamps that is originally used for forming images and a light-receiving part of a home position sensor for determining the position of the paper and thus does not need a special densitometer (Japanese Patent Application No. 11-141500). The printer corrects the difference between the gray density and a reference density to correct an average density due to differences of sensitivities of the papers.
However, the C, M and Y layers of TA paper do not always have the same sensitivity characteristics. For example, the decline of the sensitivity due to drying of the C layer is larger than that of the Y layer. It is preferable to separately measure the densities of the three colors and separately correct the differences between the densities of the colors and reference densities.
For measuring the densities of C, M and Y colors with white being the reference, a plurality of light-receiving sensors that have spectral sensitivity characteristics only in R, G and B areas. But, if a plurality of light-receiving sensors with color separation filters is used, the cost is high. On the other hand, if the densities of test patterns of C, M and Y colors by one light-receiving sensor that has a broad spectral sensitivity for R, G and B colors, signal-to-noise ratios are low and the densities of C, M and Y colors can not be found even though the ratios of the light amounts of C, M and Y colors to that of white.
The case in which the density of M color in the G light area is measured with a light-receiving sensor that has a spectral sensitivity characteristics in wavelength areas of R, G and B colors will now be explained.
FIG. 14(A) shows spectral reflection factors of test patterns (solid line) of M color and white (dotted line). FIG. 14(B) shows an emission spectrum of a light source used for density measurement, and FIG. 14(C) shows spectral sensitivity characteristics of a light-receiving sensor used for the measurement of amounts of reflected lights. FIG. 14(D) shows reception spectrums of M color and white of the light-receiving sensor.
Originally, the density of M color is represented by the logarithm of the ratio of the light amounts in wavelength area of approximately 500 nm through 600 nm in FIG. 14(D). Spectrums of blue light with wavelength of approximately 400 nm through 500 nm and red light with wavelength of approximately 600 nm through 680 nm are unnecessarily received. As a result, the signal-to-noise ratios lower and the densities of C, M and Y colors can not be measured.